add --uses-library option to manifest_fixer.py

`--uses-library foo` adds <uses-library android:name="foo"
android:required="true"/> element to the manifest.

When there is already a uses-library element with the same
name, then the hand-written element is respected.

Test: m -j
Test: build/soong/scripts/manifest_fixer_test.py

Change-Id: Ia5e989d3c3c3fba7cc9015a6d16dac70464282a8