Ensure task scheduler is already initialized in ledger lib

[yrliou]

brave/brave-core#1800 added a task runner in ledger lib.
Task scheduler used in the ledger lib is already initialized by chromium code in desktop case but not in iOS.
We would need to ensure task scheduler is initialized before creating the task runner in ledger lib to avoid crashes on iOS.

[btlechowski]

Verification passed on

Brave	0.63.14 Chromium: 73.0.3683.75 (Official Build) dev(64-bit)
Revision	909ee014fcea6828f9a610e6716145bc0b3ebf4a-refs/branch-heads/3683@{#803}
OS	Ubuntu 18.04 LTS

Used test plan from brave/brave-core#1835

Verification passed on

Brave	0.63.15 Chromium: 73.0.3683.75 (Official Build) dev (64-bit)
Revision	909ee014fcea6828f9a610e6716145bc0b3ebf4a-refs/branch-heads/3683@{#803}
OS	Windows 10 OS Build 17134.523

• Verified the test plan from Ensure task scheduler used by ledger lib is initialized before creati… brave-core#1835

Verified passed with

Brave	0.63.21 Chromium: 73.0.3683.75 (Official Build) dev(64-bit)
Revision	909ee014fcea6828f9a610e6716145bc0b3ebf4a-refs/branch-heads/3683@{#803}
OS	Mac OS X

• Verified the test plan from Ensure task scheduler used by ledger lib is initialized before creati… brave-core#1835