Remove strict mode violation from value to string coercion

The legitimate uses of value to string coercion, including the
interaction between lax attribute formatting and AAPT type inference,
combined with the low likelihood of unintentional coercion and low cost
of most string coercions, makes the value to string coercion violation
overkill.

Bug: 21563086
Change-Id: I7892e776d3e5479fcba507749b074c2abdf1b781