Support multi font update internally.

This CL make UpdatableFontDir support multi font update in transaction.
The public / shell API is TBD.

Bug: 179103383
Test: atest CtsGraphicsTestCases:FontManagerTest
Test: atest FrameworksServicesTests:PersistentSystemFontConfigTest
Test: atest FrameworksServicesTests:UpdatableFontDirTest
Test: atest UpdatableSystemFontTest
Change-Id: If9474a8ab81fe194b2d76080a4b066131fcd9e44