Invoke BlobStoreManagerService.onStart() asynchronously.

Bug: 148299993
Test: atest cts/tests/BlobStore/src/com/android/cts/blob/BlobStoreManagerTest.java
Test: tradefed.sh run google/template/local --template:map \
      test google/continuous/boot-successive  \
      --no-first-boot --successive-boot --boot-count 10
Test: tradefed.sh run google/template/local --template:map \
      test google/continuous/boot-successive \
      --no-first-boot --successive-boot --boot-count 2 --granular-boot-info
Change-Id: I99e3ca1c28a47e6a855c6d61295e9ddb4b4ac29c