Safely obtain dynamic color styleables when context theme is null

Fall back to creating a new theme via resources.newTheme() if context.theme is null when resolving dynamic color styles.

Test: ./gradlew :compose:material3:material3:connectedAndroidDeviceTest -Pandroid.testInstrumentationRunnerArguments.class=androidx.compose.material3.ColorSchemeTest#dynamicColorSchemes_unthemedContext_resolveCorrectly
Fixes: 552935916
Relnote: N/A
Change-Id: I1c0501f7912161485c386286fc70218af86839cf